1. Field of the Invention (Technical Field)
This invention relates to cooling of electronic assemblies.
Since the successful introduction of integrated circuits (ICs), there has been a steady progression toward larger IC devices to permit inclusion of a greater number of functions on the IC. The result is that the heat produced by these larger semiconductor devices is not adequately removed by either natural air convection nor by powered ventilation, such as a fan.
Convection cooling is performed by use of a heat sink that is directly applied to the semiconductor device. It is not desirable to use a bonding process, such as an adhesive, because of the permanent nature of such attachment; a semiconductor device may be discarded if malfunctioning and it is not desirable to throw away the heat sink too. It has therefore been found desirable to attach the heat sink to the chip or device by mechanical means such as a retainer clip. The typical heat sink currently in use comprises a flat plate generally the same size as the device, with a plurality of radiation tins extending upwardly. The heat sink is commonly fabricated from aluminum and is extruded and subsequently machined.
A new development in the electronic assembly art particularly for microprocessor semiconductor devices, utilizes a cartridge which contains a printed circuit board on which the microprocessor is mounted. The cartridge may be formed of plastic except for one major surface which may be constructed of metal so as to provide a thermal interface or plate for attachment of a heat sink. At least one major microprocessor supplier, Intel, has established a specification for the cartridge including the thermal plate. To facilitate the proper attachment of the heat sink to the cartridge thermal plate, the thermal plate is provided with multiple openings to permit the engagement of a heat sink retaining clip, or multiple clips.
The present invention will be described in its preferred embodiment in connection with the above-described microprocessor cartridge. However, it should be understood that the heat sink assembly, i.e. the heat sink and retainer clip, may also be used directly with a semiconductor device mounted directly on a printed circuit board or a socket.
2. Background and Prior Art
A wide variety of heat sink assemblies and retainer clips are available in the prior art. For example, the applicant's earlier U.S. Pat. No. 5,600,540 shows a heat sink and retainer for electronic integrated circuits comprising a two-piece retainer clip particularly adapted for use in connection with a semiconductor device mounted directly on a socket which in turn is attached to a printed circuit board. Another device is shown in applicant's earlier U.S. Pat. No. 5,208,731 which is a single-piece retainer clip for a similar chip, socket, and heat sink assembly. Other devices are shown in Bright U.S. Pat. Nos. 5,448,449 and 4,716,494 the former showing both one or two piece retainer clips for securing a heat sink to a semiconductor chip which is mounted in a socket and the latter showing a similar assembly with a different type of heat sink. Other prior art includes Clemens U.S. Pat. No. 5,371,652 which shows several types of retainer clips for attaching a heat sink directly to a semiconductor chip.
In all of the above-described devices, a common problem is the cost of the retainer clip and, much more importantly, the labor required to properly attach the retainer clip to secure the heat sink to the semiconductor chip and/or socket. It must be understood that when a semiconductor device such as a microprocessor is used in a personal computer, literally millions of chips, sockets, heat sinks, and retainer clips are used in these ubiquitous products. Thus, the time in which it takes to properly secure the heat sink in position and to attach it to the semiconductor device or socket becomes a critical consideration in the cost-effectiveness of a retainer clip. Other problems include design of a retainer clip so that there is a positive and easily recognized proper engagement of the retainer clip. If the means for attaching the retainer clip to the socket and/or chip is not positive in a sense that a retainer clip can only be attached or detached, with no middle ground, the assembly may not be attached yet that may not be noticeable to the technician or assembler performing that operation. The result is that when the computer or other electronic device is shipped, and is subjected to various forces, shaking, vibration, and other environmental shipping conditions the retainer clip may be unloosed with the result that the heat sink becomes disengaged and can ruin other components within the electronic device as the device is continued to be roughly handled in shipping. Even if other components are not damaged, with the heat sink dislodged, there being little likelihood that the user will be aware of such problem, the semiconductor device can be easily overheated and damaged or destroyed.
Still another problem with prior art devices is the cost of the retainer clip itself. Many proposed prior art devices require complex machining operations which greatly increase the price of the clip. It is highly desirable to make the clip using only the simplest metal forming devices such as a stamping machine. Wires are particularly unsuitable for the construction of retainer clips; although they are simple in appearance, the bending and forming of wire is a more complex and expensive forming operation than simple stamping. Other prior art devices use molded plastic and/or metal parts that must be cast or forged which again are more expensive metal forming operations.
Another problem with prior art retainer clips again relates to the rough handling that occurs during shipping of electronic devices which contain a semiconductor device, heat sink and retainer clip. It is of course desirable in all retainer clips that a relatively uniform and large pressure be applied to the heat sink so that the lower flat base surface is in intimate heat-conducting engagement with the upper surface of the chip, or with the module plate (see above). This pressure creates a frictional engagement between these two surfaces. However, with a sharp blow in the plane of the surfaces, such as the acceleration that may be experienced during some sudden starting or stopping of the electronic device in which this assembly is contained, the heat sink may slide relative to the chip and then strike the retainer clip holding means so as to dislodge the clip and again allowing the heat sink to be separated from the chip and heat sink assembly. Even partial movement of the heat sink base relative to the semiconductor device so that the heat sink is still in engagement with the semiconductor device but is displaced will affect the efficiency of the heat transfer from the device to the heat sink.